A conventional mobile communication system of the LTE (Long Term Evolution) scheme specified by the 3GPP is configured to communicate between a mobile station UE and a radio base station eNB using a certain key.
The certain key includes, for example, a key KRRC—Ciph used for “Ciphering” in an RRC protocol, which is a C-plane protocol between the mobile station UE and the radio base station eNB (Access Stratum, AS), a key KRRC—IP used for “Integrity Protection” in the RRC protocol, and a key KUP—Ciph used for “Ciphering” in a U-plane protocol between the mobile station UE and the radio base station eNB (Access Stratum, AS) and the like. These certain keys are generated using a first key KeNB.
Using the same key as any of the certain keys and the first key KeNB for a long time is not preferable because it makes the system's security vulnerable. For this reason, a procedure for updating such a certain key or a first key KeNB during handover is devised by the 3GPP.
Here, operations of a radio base station (Target eNB) managing a re-establishment target cell to acquire a first key KeNB** used for generating a certain key in the re-establishment procedure for a mobile station UE are described referring to FIG. 8.
As shown in FIG. 8, first, a radio base station (Source eNB) managing a re-establishment source cell generates an intermediate key KeNB*, based on a stored first key KeNB, a parameter “Next Hop”, a parameter “Handover Type” representing a handover type and a parameter “Target PCI” representing identification information of a handover target cell.
Secondly, the radio base station (Source eNB) managing a re-establishment source cell transmits the generated intermediate key KeNB* to the radio base station (Target eNB) managing the re-establishment target cell.
Thirdly, the radio base station (Target eNB) managing the re-establishment target cell generates the first key KeNB** used for generating a certain key at the radio base station (Target eNB) managing the re-establishment target cell, based on the received intermediate key KeNB* and “C-RNTI (Call Radio Network Temporary ID)” allocated by the re-establishment target cell.